Communication system

ABSTRACT

A communication system includes a control unit, a switch and a diversity hand-over processing unit connected to the control unit and the switch for receiving both a first communication signal received from a radio base station which outputs the hand-over instruction, and a second communication signal received from another radio base station to which the communication signal will be hand-over so as to output one of the first and second communication signals to the associated interface circuit. The diversity hand-over processing unit transmits the communication signal received through the radio base station from the mobile station on the reception side for communication to both the interface circuit associated with the radio base station as a source of hand-over and the interface circuit associated with the radio base station as a destination of hand-over.

The present application is a continuation of application Ser. No.09/906,677, filed Jul. 18, 2001; now U.S. Pat. No. 6,795,414; which is acontinuation of application Ser. No. 09/638,028, filed Aug. 15, 2000,now U.S. Pat. No. 6,289,008; which is a continuation of application Ser.No. 09/411,655, filed Oct. 4, 1999, now U.S. Pat. No. 6,178,165; whichis a continuation of application Ser. No. 08/970,159, filed Nov. 13,1997, now U.S. Pat. No. 6,049,534, the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates in general to a communication system forcarrying out a hand-over which is requested from a radio base stationdue to the movement of a mobile station, and more particularly to apacket communication system for managing packet signals and anAsynchronous Transfer Mode (ATM) system for managing cell signals.

In recent years, high reliability has been required for a publiccommunication network for transmitting data relating to voice andimages. Nowadays having entered the multimedia era, data is transmitted,for example, utilizing mobile communications. Hence for mobilecommunications, high reliability has been required when carrying out thehand-over of calls.

A discussion of the hand-over of call is provided, for example, by thereference “Mobile Communication Network, the 12th series of InformationNetwork”, by Mitsuo Yokoyama, Shokoudou.

SUMMARY OF THE INVENTION

In the light of the foregoing, it is an object of the present inventionto provide a communication system which is capable of realizinghand-over of calls in such a way that during the hand-over resultingfrom the movement of a mobile station, interruption of communicationdoes not occur.

It is another object of the present invention to provide a communicationsystem which is capable of realizing hand-over in such a way thatinterruption of communication can be prevented from occurring only byadding a trunk function thereto without changing the circuitconfiguration of the existing communication system.

It is still another object of the present invention to provide acommunication system which can be used as an Asynchronous Transfer Mode(ATM) exchange system for managing cell signals as well as a packetexchange system for managing packet signals.

In order to attain the above-mentioned objects, the present inventionprovides a communication system which includes a plurality of interfacecircuits through which communication signals are transmitted between amobile station and a plurality of radio base stations via transmissionpaths. A switch is provided for assigning the communication signals tothe interface circuits in such a way that the communication signals fromthe interface circuits are distributed in accordance with headerinformation included in each communication signal indicating how thecommunication signal is to be transmitted from the radio base stationsto the mobile station via the transmission paths. A control unit,connected to both the interface circuits and the switch, receives ahand-over instruction, which has been issued from the radio base stationin accordance with the movement of the mobile station, from theassociated interface circuit. A diversity hand-over processing unit,connected to both the control unit and the switch, receives both a firstcommunication signal received from the radio base station, as a sourceof hand-over, which has output a hand-over instruction, and a secondcommunication signal received from the radio base station as thedestination of hand-over, which will be handed-over, in accordance withthe hand-over instruction to output one of the first and secondcommunication signals to the associated interface circuit. The diversityhand-over processing unit also transmits the communication signal whichhas been received from the mobile station on the reception side forcommunication via the radio base station to both the interface circuitassociated with the radio base station as source of hand-over and theinterface circuit associated with the radio base station as destinationof hand-over.

The above-mentioned switch can be used in both an Asynchronous TransferMode (ATM) switch for managing cell signals and a packet switch formanaging packet signals.

The above-mentioned diversity hand-over processing unit includes aninstruction processing unit which operates in accordance with aninstruction outputted from the control unit, a synchronous selectionunit, and a signal duplicating unit.

The asynchronous selection unit receives, via the switch, when a servicearea is changed from the radio-base station as source of hand-over tothe radio base station as destination of hand-over, both thecommunication signal from the interface circuit associated with thetransmission path extending to the radio base station as source ofhand-over and the communication signal from another interface circuitassociated with the transmission path extending to the radio basestation as destination of hand-over, in accordance with an instructionissued from the instruction processing unit. The synchronous selectionunit also synchronizes both the two communication signals with eachother, and outputs one of the two communication signals which is moreexcellent in quality to the interface circuit associated with the radiobase station which communicates with the mobile station on the receptionside for communication. The signal duplicating unit receives acommunication signal from the interface circuit associated with thetransmission path extending to the radio base station which communicateswith the mobile station on the reception side for communication via theswitch, and transmits the communication signal thus received to both theinterface circuit associated with the radio base station as a source ofhand-over and the interface circuit associated with the radio basestation as destination of hand-over. The signal duplicating unit alsotransmits the communication signal from the interface circuit associatedwith the radio base station which communicates with the mobile stationon the reception-side for communication to the radio base station asdestination of hand-overthrough that interface circuit.

The synchronous selection unit includes a first switch interface circuitfor receiving, through an input port, both the communication signal fromthe radio base station as source of hand-over and the communicationsignal from the radio base station as destination of hand-over. A firstselector is provided for receiving the communication signals todistribute, and in response to the communication signals, distributingthese communication signals in accordance with the header informationwhich the respective communication signals have, and a plurality ofbuffers are provided for temporarily storing both the communicationsignal from the radio base station as source of hand-over and thecommunication signal from the radio base station as destination ofhand-over which communication signals have been outputted from the firstselector. A synchronous circuit synchronizes the communication signalswith each other which have been stored temporarily in the plurality ofbuffers. A selection circuit orders the respective communication signalsin accordance with a predetermined algorithm. A header conversioncircuit, having a header conversion table, converts the headerconversion table, in accordance with the header information which thecommunication signals have, in such a way that the communication signalsare directed from the switch to the mobile station on the reception sidefor communication through the interface circuit. A second selectorselects among the communication signals from the header conversioncircuit, and a second switch interface circuit outputs the communicationsignal from the second selector to the switch through an output port.

The signal duplicating unit includes a third switch interface circuitfor receiving through an output port the communication signal outputfrom the mobile station on the reception side for communication from theswitch and a first signal processing unit, having a header conversiontable, for changing the header conversion table in such a way that thecommunication signal of interest is directed to the radio base stationas source of hand-over. A second signal processing unit, having a headerconversion table, is provided for changing the header conversion tablein such a way that the communication signal of interest is directed tothe radio base station as destination of hand-over and first and secondbuffers are provided for temporarily storing the respectivecommunication signals from the first and second signal processing units.A selector selects among the communication signals from the first andsecond buffers, and a fourth switch interface circuit outputs through anoutput port the communication signal from the selector to the switch.

By adopting the above-mentioned circuit configuration, during thehand-over, the diversity hand-over processing unit receives both thecommunication signals from the radio base station as source of hand-overand the radio base station as destination of hand-over which radio basestations are associated with the mobile station on the transmission sidefor communication, and then one of the two communication signals whichis more excellent in quality is transmitted from the switch to themobile station on the reception side for communication. In addition, thediversity hand-over processing unit transmits the communication signalfrom the mobile station on the reception side for communication to boththe radio base station on the origin of hand-over and the radio basestation on the destination of hand-over, and in accordance with thecompletion of the hand-over, the communication is established betweenthe mobile station on the transmission side for communication and themobile station which will receive the hand-over instruction and whichwill receive the communication signal. Therefore, during the hand-over,no interruption of the communication occurs at all.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects as well as advantages of the presentinvention will become clear by the following description of thepreferred embodiments of the present invention with reference to theaccompanying drawings, wherein:

FIG. 1 is a block diagram showing a circuit configuration of oneembodiment of a communication system according to the present invention;

FIG. 2 is a schematic view useful in explaining the relation amongmobile stations, base stations and a communication system;

FIG. 3 is a block diagram useful in explaining configuration of aninterface;

FIG. 4 is a schematic view showing a structure of header information;

FIG. 5 is a schematic view useful in explaining the state from thetransmission side for communication to the reception side forcommunication when carrying out the diversity hand-over of the presentinvention;

FIG. 6 is a schematic view useful in explaining the state from thereception side for communication to the transmission side forcommunication when carrying out the diversity hand-over of the presentinvention;

FIG. 7 is a block diagram showing a configuration of a diversityhand-over processing unit of the present invention;

FIG. 8 is a flow chart showing the sequence of the diversity hand-overof the present invention; and

FIG. 9 is a block diagram showing a configuration of another diversityhand-over processing unit of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will hereinafter bedescribed in detail with reference to the accompanying drawings.

FIG. 1 shows a configuration of an ATM communication system for use inrealizing the diversity hand-over according to the present invention.

An ATM communication system 1 includes a plurality of interface circuits11 a to 11 n through which the cells on transmission paths 14 a to 14 nare transmitted, an ATM switch 10 and a control unit 12, and also adiversity hand-over processing unit 15 is connected as a trunk to theATM switch 10. This trunk is the well known general unit and isinstalled in the I/O (Input/Output) lines in order to carry out themonitor and control of the transmission paths. The diversity hand-overprocessing unit 15 includes: a synchronous selection unit 16 forcarrying out, in the diversity hand-over, the selection between the cellcoming from a base station 2-1 or a base station 2-2 as a source ofhand-over, for example, from which the hand-over command has beenoutputted and the cell coming from a base station 2-n as a destinationof hand-over which will receive the hand-over command, and also fortransmitting the cell thus selected to the reception side forcommunication; and a cell duplicating unit 17 for distributing the cellcoming from the reception side for communication between the basestation 2-1 from which the hand-over command has been outputted and thebase station 2-2 which will receive the hand-over command. An interfacecircuit 11 m is connected to another ATM communication system which isconnected to a network 4 (FIG. 2).

Now, the description will hereinbelow be given with respect to thegeneral hand-over function with reference to FIG. 2. Firstly, a mobilestation 3-1 on the transmission side for communication communicates witha mobile station 3-2 on the reception side for communication. The mobilestation 3-1 is located within the service area of a base station 2-1,and hence the communication signal is transmitted to the network 4through the base station 2-1. On the other hand, the mobile station 3-2is located within the service area of a base station 2-n, and hence thecommunication signal is transmitted to the network 4 through the basestation 2-n. Thus, the mobile station 3-2 communicates with the mobilestation 3-1 through both the ATM communication system 1 and the basestation 2-1. The cells from the base stations 2-1, . . . , 2-n aretransmitted to the ATM communication system 1 which carries out therouting therefor. In FIG. 2, the cell which has reached the ATMcommunication system 1 from the mobile station 3-1 through the basestation 2-1 is then transmitted so as to be directed to the mobilestation 3-2 through the base station 2-n by the ATM communication system1. In addition, conversely, the cell which has reached the ATMcommunication system 1 from the mobile station 3-2 through the basestation 2-n is then transmitted so as to be directed to the mobilestation 3-1 through the base station 2-1 by the ATM communicationsystem.

If the mobile station 3-1 is moved, then the mobile station 3-1 leavesthe service area of the base station 2-1 and then enters into theservice area of the base station 2-2. At this time, the communication ofthe mobile station 3-1 is changed from the base station 2-1 to the basestation 2-2 and the communication thereof continues through the basestation 2-2. That is, in the ATM communication system 1, thecommunication path for the mobile station 3-1 is switched from the basestation 2-1 to the base station 2-2. In such a way, the so-calledhand-over is carried out.

Incidentally, by the source of hand-over is meant a hand-over that isgenerated with the mobile station 3-1 as a source when the mobilestation 3-1 is moved to enter from the service area of the base station2-1 into the service area of the base station 2-2. On the other hand, bythe destination of hand-over is meant a hand-over that will be carriedout from now on when the mobile station 3-1 is moved to enter into theservice area of the base station 2-2.

Next, the description will hereinbelow be given with respect to theinterface circuit 11 a with reference to FIG. 3. Incidentally, each ofother interface circuits 11 b to 11 n, 11 m has also the sameconfiguration as that of the interface circuit 11 a. Cell coming overthe transmission path 14 a terminates in a physical layer at a physicallayer processing unit 20 to be transmitted to a cell layer processingunit 21 which carries out the header conversion of the cell, i.e.,changes the contents of a header conversion table 22 which the celllayer processing unit 21 has.

Now, the header conversion table 22 will hereinbelow be described withreference to FIG. 4. The header conversion table 22 consists of areception side header 30, a header 31 within the unit and a transmissionside header 32 which are set at every communication path. At the timewhen the cell has reached the interface circuit 11 a shown in FIG. 3,the reception side header 30 which the coming cell has is duplicated inthe reception side header 30, and on the basis of the reception sideheader 30, both the header 31 within the unit and the transmission sideheader 32 are produced to be applied to the coming cell.

The cell which has been subjected to the header conversion in such a wayis then transmitted to the ATM switch 10 through a switch interface 23.On the other hand, the cell which has reached the interface circuit 11 afrom the ATM switch 10 is then transmitted to the cell layer processingunit 21 through the switch interface 23 to be transmitted from thephysical layer processing unit 20 to the transmission path 14 a. All thephysical layer processing unit 20, the cell layer processing unit 21,the switch interface 23, a RAM 24 and a control unit interface 26 whichare provided within the interface circuit 11 a are connected to oneanother through an MPU bus 27, and also are controlled by an MPU 25which is also connected to the MPU has 27. The RAM 24 is a processingarea of the MPU 25, and also the MPU 25 is connected to a control systemtransfer path 13 through the control unit interface 26 and is controlledby the control unit 12.

Next, the description will hereinbelow be given with respect to theoperation of the ATM communication system 1 for carrying out thediversity hand-over with reference to FIG. 5. The description willhereinbelow be given with respect to the operation of the communicationpath extending from the mobile station 3-1 on the transmission side forcommunication up to the mobile station 3-2 on the reception side forcommunication through the base station 2-1, the ATM communication system1 and the base station 2-n, with reference to FIG. 2. Incidentally, thiscommunication path is referred as the forward communication path X, whenapplicable. In FIG. 5, an arrow indicated by a heavy line represents theflow of the cell, and an arrow indicated by a dotted line representsthat the cell is in the state of being able to be transmitted.

A part (a) of FIG. 5 shows the state in which the mobile station 3-1communicates with the mobile station 3-2 through the base station 2-1,the ATM communication system 1 and the base station 2-n. That is, thereis shown the state in which the ATM switch 10 is switched in such a waythat the cell from the base station 2-1 as a source of hand-over istransmitted to the base station 2-n through the interface circuit lla,the ATM switch 10 and the interface circuit 11 n.

In a part (b) of FIG. 5, when the mobile station 3-1 has been moved fromthe service area of the base station 2-1 to enter into the service areaof the base station 2-2, if a request of the hand-over from the mobilestation 3-1 is received by the ATM communication system 1 through thebase station 2-1, then the contents of the header conversion table 22which the interface circuit 11 a connected to the base station 2-1 hasare changed so that the cell from the interface circuit 11 a istransmitted to the synchronous selection unit 16 (which will bedescribed in detail later). In the synchronous selection unit 16, thecontents of the header conversion table 22 a which a header conversioncircuit 50 a (which will be described later) of the synchronousselection unit 16 has are changed so that the cell which has beenreceived from the base station 2-1 as a source of hand-over can betransmitted to the interface circuit 11 n. At this time, the cell iscontinuously transmitted from the interface circuit 11 a connected tothe base station 2-1 as a source of hand-over to the interface circuit11 n.

In a part (c) of FIG. 5, when the setting of the header conversion table22 a has been completed, since the mobile station 3-1 starts alreadyentering into the service area of the base station 2-2, the contents ofthe header conversion table 22 of the interface circuit 11 b having theconnection relation with the base station 2-2 are changed, so that thecell from the interface circuit 11 b is transmitted to the synchronousselection unit 16. In the synchronous selection unit 16, the contents ofthe header conversion table 22 a which the header conversion circuit 50a has are changed so that the cell from the interface circuit 11 b canbe transmitted to the interface circuit 11 n. In such a way, both thecell from the base station 2-1 as a source of hand-over and the cellfrom the base station 2-2 as a destination of hand-over are transmittedto the synchronous selection unit 16. In the synchronous selection unit16, after the cell from the base station 2-1 as a source of hand-overhas been synchronized with the cell from the base station 2-2 as adestination of hand-over, one of the two cells which has the higherreception sensitivity is selected.

In a part (d) of FIG. 5, the separation of the cell from the basestation 2-1 as a source of hand-over is carried out. For this purpose,firstly, the communication path between the base station 2-1 as a sourceof hand-over and the ATM communication system 1 is interrupted. Then,the setting of the contents of the header conversion table 22 of theinterface circuit 11 b is changed so that the cell from the base station2-2 as a destination of hand-over is transmitted to the base station 2-non the reception side for communication through the interface circuit 11n. At this time, the setting of the contents of the header conversiontable 22 a of the synchronous selection unit 16 is left as it is.

In a part (e) of FIG. 5, the control unit 12 cancels the control by thediversity hand-over processing unit 15.

In such a way, in the forward communication path X, the hand-over iscarried out without interrupting the communication established betweenthe mobile station 3-1 on the transmission side for communication andthe mobile station 3-2 on the reception side for communication.

Next, the description will hereinbelow be given with respect to theoperation, out of the execution of the diversity hand-over, of thecommunication path extending from the mobile station 3-2 on thereception side for communication to the mobile station 3-1 through thebase station 2-n, the ATM communication system 1 and the base station2-2. This communication path is referred as the backward communicationpath Y, when applicable. In FIG. 6, an arrow indicated by a heavy linerepresents the flow of the cell, and an arrow indicated by a dotted linerepresents that the cell is in the state of being able to betransmitted.

A part (a) of FIG. 6 shows the state in which the mobile station 3-2communicates with the mobile station 3-1 through the base station 2-n,the ATM communication system 1 and the base station 2-1. That is, thereis shown the state in which the ATM switch 10 is switched so that thecell from the base station 2-n on the reception side for communicationis transmitted to the base station 2-1 through the interface circuit 11n and the ATM switch 10.

In a part (b) of FIG. 6, when the mobile station 3-1 has been moved fromthe service area of the base station 2-1 to enter into the service areaof the base station 2-2, if the request of hand-over from the mobilestation 3-1 is received by the ATM communication system 1 through thebase station 2-1, then, firstly, in a cell duplicating unit 17 (whichwill be described in detail later) which the diversity hand-overprocessing unit 15 has, the contents of the header conversion table 22-1which the cell duplicating unit 17 has are set on the basis of thereception side header 30 which the received cell has so that the cellwhich has been received from the base station 2-n on the reception sidefor communication can also be transmitted to the interface circuit 11 b.At this time, the cell is continuously transmitted from the interfacecircuit 11 n connected to the base station 2-n on the reception side forcommunication to the interface circuit 11 a.

In a part (c) of FIG. 6, at the time when the contents of the headerconversion table 22-1 of the cell duplicating unit 17 have been set, thecell is duplicated in the cell duplicating unit 17 and the resultantcells are respectively transmitted to the interface circuit 11 a and theinterface circuit 11 b. That is, the so-called multicasting is carriedout.

In a part (d) of FIG. 6, the contents of the header conversion table 22of the interface circuit 11 n are changed, so that the cell from thebase station 2-n

on the reception side for communication can be transmitted to theinterface circuit 11 b. As a result, the cell from the base station 2-non the reception side for communication has been transmitted to the basestation 2-2 as a destination of hand-over through the interface circuit11 b.

In a part (e) of FIG. 6, the control by the diversity hand-overprocessing unit 15 is canceled.

In such a way, in the backward communication path Y as well, thehand-over is carried out without interrupting the communicationestablished between the mobile station 3-1 on the transmission side forcommunication and the mobile station 3-2 on the reception side forcommunication.

FIG. 7 is a block diagram showing a configuration of the above-mentioneddiversity hand-over processing unit 15. In the diversity hand-overprocessing unit 15, both the cell duplicating unit 17 and thesynchronous selection unit 16 are connected to an MPU bus to which anMPU 53, a RAM 52 having the processing area of the MPU 53, and a controlunit interface circuit 54 for use in the interface with the control unit12 which the ATM communication system 1 has are further connected, sothat the MPU 53 controls both the cell duplicating unit 17 and thesynchronous selection unit 16 in accordance with the instructionoutputted from the control unit 12.

The cell duplicating unit 17 has one input port and one output port forinput/output of the cell to/from the ATM switch 10 and when the cellwhich has been transmitted by the mobile station 3-2 on the receptionside for communication through the ATM switch 10 is received by a switchinterface circuit 40-1 through the base station 2-n, distributes thecell thus received between a cell processing unit 42-1 and a cellprocessing unit 42-2. Incidentally, while in this example, the cell isdistributed between the two cell processing units, in order to improvethe processing efficiency, two or more cell processing units may beprovided. In both the cell processing units 42-1 and 42-2, the contentsof each of header conversion tables 43-1 and 43-2 are converted so thatthe state shown in the part (c) of FIG. 6 can be obtained, i.e., thecell is transmitted to both the interface circuits 11 a and 11 b. Thecontents of each of the header conversion tables 43-1 and 43-2 are thesame as those shown in FIG. 4. The converted headers are given to thecell which are transmitted to buffers 44-1 and 44-2, respectively. Aselector 45 reads out the cells from the buffers 44-1 and 44-2 and thentransmits the cells thus read out to a switch interface circuit 41-1 tooutput the cells to the ATM switch 10.

The synchronous selection unit 16 has similarly to the cell duplicatingunit 17, an input port and an output port for use in input/output of thecell to/from the ATM switch 10. At the time when the cell from the ATMswitch 10 has been received by a switch interface circuit 40-2, aselector 46 transmits both the cell coming from the source of hand-over(the base station 2-1) and the cell coming from the destination ofhand-over (the base station 2-2) to paired buffers 47 a-1 and 47 a-2. Asynchronous circuit 48 a compares the sequence numbers, which are shownin the respective headers of the cell from the source of hand-over andthe cell from the destination of hand-over, with each other, andsynchronizes both the cells to send the cells to a selection circuit 49a. The selection circuit 49 a selects one of the cells in accordancewith a predetermined algorithm and transmits the selected cell to aheader conversion circuit 50 a. The header conversion circuit 50 a has aheader conversion table 22 a, the format of which is the same as thatshown in FIG. 4. In the header conversion circuit 50 a, the contents ofthe header conversion table 22 a are changed so that the cell ofinterest can be directed to the interface circuit 11 n associated withthe mobile station 3-2 on the reception side for communication throughthe base station 2-n. In a selection 51, the scheduling is carried outso that the one of the two cells which is more excellent in quality isselected, and the selected cell is transmitted to the ATM switch 10through the switch interface circuit 41-2. In such a way, if theinstruction has been outputted from the control unit 12 to an MPU 53,then the MPU 53 carries out the above-mentioned control. Incidentally,as shown in FIG. 7, a plurality of paired buffers 47 a-1 and 47 a-2, 47n-1 and 47 n-2, a plurality of synchronous circuits 48 a, . . . , 48 n,a plurality of selection circuits 49 a, 10-49 n, and a plurality ofheader conversion circuits 50 a, . . . , 50 n can be provided, whichenables the processing efficiency to be improved.

FIG. 8 shows the sequences of the diversity hand-over processing whichfunctions as a trunk. The sequences indicated by reference symbols (a)to (e) in FIG., 8 correspond to the parts (a) to (e) of FIG. 5 and FIG.6. Firstly, a request of hand-over from the mobile station 3-1 isreceived by the control unit 12 of the ATM communication system 1 (Step60). Then, the control unit 12 instructs the diversity hand-overprocessing unit 15 to activate the diversity hand-over (Step 61). Thediversity hand-over processing unit 15 synchronizes both the cell comingfrom the source of hand-over and the cell coming from the destination ofhand-over with each other, and then selects one of the cells in thesynchronous selection unit 16 to carry out the header conversion (Step80). At the same time, the cell duplicating unit 17 receives the cellfrom the reception side for communication through the base station 2-n,and then sets the header so that the cell is transmitted to both theinterface circuits 11 c having the connection relation with the sourceof hand-over and the interface circuit 11 b having the connectionrelation with the destination of hand-over (Step 81). After completionof the setting of the header, the data relating to the settingcompletion is reported to the control unit (Step 62).

Next, the control unit 12 instructs the interface circuit 11 a in thesource of hand-over, the interface circuit 11 b in the destination ofhand-over and the interface circuit 11 n on the reception side forcommunication. Then, in the interface circuits 11 a and 11 b, thesetting of the contents of the header conversion tables 22 is changed sothat the cell over the forward communication path X is transmitted tothe diversity hand-over processing unit 15 (Step 82 and 83), and in theinterface circuit 11 n on the reception side for communication, thecontents of the header conversion tables 22-1 and 22-2 are changed sothat the cell over the backward communication path Y is transmitted tothe diversity hand-over processing unit 15 (Step 84). The report of thecompletion of these processings is received by the control unit 12 (Step64).

The diversity hand-over processing unit 15 stairs receiving the cell andcarries out the synchronous processing with respect to the cell over thebackward communication path Y so that the diversity reception of thecell is established (Step 85). When the processing becomes the diversityreception state of the cell, this is reported to the control unit 12(Step 65).

When the report of establishment of the diversity reception of the cellhas been received by the mobile station 3-1 (Step 66), firstly, theinstruction of completion of the diversity reception of the cell isoutputted to the interface circuit 11 a as a source of hand-over (Step67). Then, the interface circuit 11 a cancels the setting of the headerconversion table 22 for the forward communication path X (Step 86).Then, the canceling report is sent to the control unit (Step 68).

Next, the instruction of completion of the diversity reception isoutputted to both the interface circuit 11 b as a destination ofhand-over and the interface circuit 11 n on the reception side forcommunication (Step 69). The interface circuit 11 b changes the contentsof the header conversion table 22 for the cell over the forwardcommunication path X so that the cell of interest is directed to theinterface circuit 11 n on the reception side for communication (Step87). On the other hand, the interface circuit 11 n changes the contentsof the header conversion table 22 for the cell over the backwardcommunication path Y so that the cell of interest is directed to theinterface circuit 11 b as a destination of hand-over (Step 88). As aresult, the diversity transmission/reception of the cell has beencompleted. Thereafter, the completion report is sent to the control unit12.

Finally, in order to release the diversity hand-over processing unit 15,the instruction of completion of the diversity hand-over is outputted tothe 5 diversity hand-over processing unit 15 (Step 71). Then, thediversity hand-over processing unit 15 releases the header conversiontable 22 a for the forward communication path X, and the headerconversion tables 22-1 and 22-2 for the backward communication path Y(Step 89). Then, the completion report is sent to the control unit 12(Step 72). In such a way, the diversity hand-over has been completed.

FIG. 9 shows an example of other ATM communication system 1 including adiversity hand-over processing circuit 15. The feature of this diversityhand-over processing circuit 15 is that the input ports between thesynchronous selection unit 16 and the cell duplicating unit 17, and theATM switch 10, and the output ports between the synchronous selectionunit 16 and the cell duplicating unit 17, shown in FIG. 1, are arrangedinto one input port and one output port, respectively. For this reason,selectors 15 a and 15 b are respectively provided in the synchronousselection unit 16 and the cell duplicating unit 17 so that the celloutput from the ATM switch 10 is supplied to the selector 15 a fromwhich the cell output is in turn supplied to both the synchronousselection unit 16 and the cell duplicating unit 17. In addition, thecell outputs from the synchronous selection unit 16 and the cellduplicating unit 17 are supplied to the selector 15 b from which thecell is in turn supplied to the ATM switch 10. As a result, the numberof ports is reduced.

In the case where as for the ATM switch 10, one having the multicastingfunction is employed, if the number of a plurality of cell processingunits 42-1 and 42-2 as shown in FIG. 7 is further increased so as tomatch the number of channels of the multicasting function, then theswitch interface circuit 40-1 becomes unnecessary, and hence the cellcan be directly transmitted from the ATM switch 10 to the cellprocessing units.

While in the above-mentioned embodiments, the specific case where theATM communication system 1 has only one diversity hand-over processingunit 15, has been described, it is to be understood that a plurality ofdiversity hand-over processing units may be provided in correspondenceto the scale of the ATM communication system 1, and as a result theprocessing efficiency is improved.

In addition, while in the above-mentioned embodiments, the descriptionhas been given with respect to the specific case where the ATMcommunication system relays the cell, it should be noted that the ATMcommunication system 1 can be directly used as the packet communicationsystem. In this case, the term “cells” which are given to theconstituent elements in the description of the embodiments as well as inthe drawings can be expressed in different term “packet”.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments and the specifiedmodifications thereof, it will be understood that the various changesand other modifications will occur to those skilled in the art withoutdeparting from the scope and true spirit of the invention. The scope ofthe invention is therefore to be determined solely by the appendedclaims.

1. A communication apparatus performing a diversity hand-over from afirst radio base station to a second radio base station among aplurality of radio base stations when a radio terminal has moved from afirst location to a second location, comprising: a plurality of inputand output ports; a switch transferring communication signals from saidplurality of input ports to at least one of said plurality of outputports in accordance with routing information included in the respectivecommunication signals; and a diversity hand-over processing unitconnected to a succeeding side of said switch and including controlfunctions for receiving from a communication source both a firstcommunication signal incoming from said radio terminal through saidfirst radio base station and a second communication signal incoming fromsaid radio terminal through said second radio base station and selectingone of said first and second communication signals to be transferred toa communication destination.
 2. A communication apparatus according toclaim 1, further comprising: a plurality of interface units performingtransmission and reception of the communication signals between saidplurality of radio base stations.
 3. A communication apparatus accordingto claim 1, further comprising: a plurality of interface unitsperforming transmission and reception of the communication signalsbetween said plurality of radio base stations, and between othercommunication apparatuses.
 4. A communication apparatus according toclaim 1, wherein a plurality of said diversity hand-over processingunits are connected to the succeeding side of said switch in parallel.